1. Field of the Invention
The present invention relates to a probe structure for testing a device-under-test (DUT), such as a semiconductor device, by a burn-in test or the like, and also to a method of manufacturing the probe structure. It is to be noted throughout the instant specification that the probe structure is electrically contacted with a contact object, such as an electrode pad or a circuit pattern, which is formed on the DUT and which may be simply called an electrode.
2. Description of the Related Art
In a conventional probe structure, a hemispherically projecting bump contact is formed as a contact point for contacting with the electrode formed on the DUT, such as a semiconductor device.
As a bump contact having the conventional bump structure, it is known in the art that fine projections are intentionally formed on a surface of the bump contact so as to improve reliability of contact with the electrode (for example, Japanese Unexamined Patent Publication (A) No. H06-27141, namely, 27141/1994). By forming fine projections on the surface of the bump contact, the contact area of the bump contact with the electrode becomes large and this enables a reliable contact with the electrode. Even when an oxide film is formed on the surface of the contact object, namely, the electrode, the fine projections can break the oxide film, and give a stable contact resistance.
Another probe structure has been disclosed in Japanese Unexamined Patent Publication (A) No. H09-133711 (133711/1997) and has a structure similar to that shown in FIG. 1. Specifically, a bump contact 2 illustrated in FIG. 1 is protruded from one side or one principal surface of an insulating substrate 1. The bump contact 2 is electrically connected through a conductive portion 4 to an electrode 3 which is operable as a part of an electric circuit provided on the other side, namely, another principal surface of the insulating substrate. The bump contact 2 has a basic shape portion 2a (an inside layer), of nickel, and an intermediate layer of plated gold on a surface of the basic shape portion 2a. In addition, a surface layer 2c and fine projections 2d each of which is formed by rhodium are deposited on the surface of this intermediate layer. The surface layer 2c and the fine projections 2d may preferably be formed by the same material (rhodium) deposited by plating. The fine projections 2d are formed by controlling plating current so as to be locally protruded from the surface layer 2c. With this structure, the surface layer 2c and the fine projections 2d are combined together to form an integrated material structure without a boundary between them. As a result, the above-mentioned publication reports that fine projections are obtained which hardly come off and which are practically kept constant in configuration.
However, no disclosure is made at all in the above-referenced publications about sizes and surface roughness of the fine projections formed on the surface of the conventional bump contact. From this fact, it is difficult from the publications to know about appropriate ranges for the surface roughness specified by Rmax and Ra, a ratio of Rmax/Ra, and about a projection pitch or spacing, the projection shape including the thickness, the projection density and other configuration of projections. As a result, forming conditions are liable to fall outside of an acceptable range and to give rise to undesirable shapes of the projections. Since the forming method itself of projections makes it difficult to avoid a variation of the projection shape, the projection shape often falls outside the acceptable range. A projection shape formed outside the acceptable range results in inconveniences, such as breakaway of projections and a variation of contact resistance.
Alternatively, a method is also disclosed in Japanese Unexamined Patent Publication (A) No. H09-133711 (133711/1997) to manufacture a bump contact. The bump contact actually manufactured by the method is disadvantageous in that adhesion of the surface layer 2c and the projections 2d is weak and the projection shape or configuration is variable. This is also similar to the case where no intermediate layer 2b of plated gold is interposed between the surface layer 2c of rhodium and the projections 2d of rhodium and, as a result, the surface layer 2c and the projections 2d are formed directly on the surface of the basic shape portion 2a. 
Practically, it is confirmed that the surface layer 2c and the projections 2d of rhodium in the bump contact manufactured by the method disclosed in the aforementioned Publication have easily peeled off in a tape peeling test. This shows a low adhesion of the projections.
In the bump contact prepared by the method described in the above-publication, as projections become high, they become thinner in some cases. This easily comes off the bump surface through repetition of contact between the bump contact and the contact object, and this brings about a variation in contact resistance.
Heretofore, it is difficult to deposit the fine projections always to a constant height (a constant surface roughness) and to keep a variation of surface roughness and a projection density invariable. This sometimes results in a variation of the contact resistance between different bumps. If the contact resistance is varied among bumps, inconvenience is liable to occur on transmitting electric signals between the probe structure and the DUT. This makes it difficult to obtain accurate and reliable measurement results due to the variation of the contact resistance among the bumps. Very high contact resistance makes it difficult to transmit and receive electric signals between the probe structure and the DUT.
As a result of searching for causes of the above circumstances, it has been ascertained that presence of an inert layer under the projecting portions brings about a poor adhesion of the projecting portions, and that presence of the inert layer gives rise to a variation of the projection shape. It has also been found out that a change in current density during plating adversely affects stability of current density and also deforms the projection shape.
Provision of an intermediate layer of gold or the like under the projecting portion has been also found to lead to a poorer adhesion of the projecting portion. Provision of such an intermediate layer has a problem of a more complicated manufacturing process.
Accordingly, it is an object of the present invention to provide a probe structure which is capable of making a configuration or shape of projections uniform within a predetermined range and which therefore has excellent properties.
It is another object of the present invention to provide a method of manufacturing a probe structure which has projections with a comparatively uniform configuration.
Another object of the invention is to provide a probe structure which has projections of a good adhesion, excellent bump contacts in strength and which rarely causes breakage to occur even after repetition of contact. The probe structure can keep contact resistance substantially constant among individual bump contacts and is easy to manufacture.
The probe structure and the manufacturing method thereof of the invention have the following configurations.
A probe structure which comprises an insulating substrate having first and second principal surfaces, a bump contact protruded from the first principal surface, and an electrode electrically connected to the bump contact and operable as a part of an electrical circuit formed on the second principal surface and/or an inner side of the insulating substrate, wherein the bump contact has a surface roughness which is specified by Rmax within a range from 0.01 to 0.8 xcexcm, Ra within a range from 0.001 to 0.4 xcexcm, and a ratio of Rmax/Ra within a range from 2 to 10.
A probe structure according to configuration 1, wherein the bump contact has, on its surface, a plurality of projections which define the surface roughness and which have a projection spacing within a range from 0.1 to 0.8 xcexcm, and a projection thickness which is not smaller than one third of the projection spacing.
A probe structure which comprises an insulating substrate having first and second principal surfaces, a bump contact protruded from the first principal surface, and an electrode electrically connected to the bump contact and operable as a part of an electrical circuit formed on the second principal surface and/or an inner side of the insulating substrate, wherein the bump contact has, at least on its surface, a convex/concave layer formed by an aggregation of fine grains.
A probe structure according to configuration 3, wherein the bump contact comprises a basic shape portion of a single layer having a base surface; the convex/concave layer being formed directly on the base surface of the basic shape portion to provide the surface of the bump contact.
A probe structure according to configuration 3, wherein the bump contact has a surface roughness which is defined by Rmax within a range from 0.01 to 0.8 xcexcm, Ra within a range from 0.001 to 0.4 xcexcm, and a ration of Rmax/Ra within a range from 2 to 10.
A probe structure according to configuration 3, wherein the fine grains have a grain size within a range from 5 to 200 nm.
A probe structure according to configuration 3, wherein the convex/concave layer on the base surface has a hardness within a range from 800 to 1,000 Hk (Knoop hardness).
A probe structure according to configuration 4, wherein the basic shape portion has a smooth hemispherical projection shape and a hardness within a range from at least 100 Hk to up to 800 Hk.
A method of manufacturing a probe structure, comprising the steps of providing a basic shape portion of a bump contact on one principal surface of an insulating substrate; providing an electrode forming at least a part of an electric circuit on the other principal surface of the insulating substrate and/or in the inside thereof; electrically connecting the basic shape portion of the bump contact to the electrode forming a part of the electrical circuit; and carrying out mat plating without substantially exposing the base surface of the basic shape portion to an atmosphere.
A method according to configuration 9, further comprising the step of forming an oxidation preventing layer for preventing oxidation of the basic shape portion, prior to the step of carrying out the mat-plating the base surface of the basic shape portion.
A method according to configuration 10, wherein the oxidation preventing layer has a thickness within a range from 0.001 to 0.05 xcexcm.
A method according to configuration 9, wherein the mat plating is carried out under a current density within a range from 0.1 to 1.0 A/dm2 with the current density kept invariable.
A method according to configuration 9, wherein the material for the mat plating is rhodium.
A probe structure according to configuration 4, wherein the bump contact has a bump which is formed by the basic shape portion of nickel alone or a nickel alloy, and a mat-rhodium-plated layer on the base surface of the basic shape portion without any inert layer interposed between the basic shape portion and the mat-rhodium-plated layer the probe structure being used for a burn-in test.
A probe structure according to configuration 4, wherein the bump contact has a bump formed by the basic shape portion of nickel alone or a nickel alloy and both a gold strike plating layer and a mat rhodium plating layer on the base surface of the basic shape portion; the probe structure being used for a burn-in test.
According to configuration 1, by using a surface roughness of the bump contact specified by Rmax within a range from 0.01 to 0.8 xcexcm, Ra within a range from 0.001 to 0.4 xcexcm, and a ratio of Rmax/Ra within a range from 2 to 10, it is possible to achieve an excellent durability against repeated contact with the device-under-test (DUT) and maintain a stable contact resistance.
Herein, Rmax and Ra are defined by the Japanese Industrial Standard (JIS B0601). Specifically, Rmax is the above-mentioned maximum height (the distance from a highest peak to a lowest valley while Ra is the above-mentioned center-line-mean roughness (the average of an absolute value of a deviation from a center line of a roughness curve to the roughness curve).
When the contact object is an aluminum electrode, an oxide film is usually formed into a thickness within a range from 0.01 to 0.1 xcexcm. A substantial roughness in a roughened state is preferably defined by an Rmax within a range of from 0.01 to 0.8 xcexcm and an Ra within a range of from 0.001 to 0.4 xcexcm. This is because the defined roughness and the thickness on this level are sufficiently enough to break the oxide film and to avoid damages of the electrode as a whole. A surface roughness represented by Rmax less than 0.01 xcexcm and Ra less than 0.001 xcexcm gives only an insufficient effect of breaking the oxide film of a metal on the contact object even when the surface is brought into contact with the object. With a surface roughness represented by Rmax over 0.8 xcexcm and Ra over 0.4 xcexcm, in contrast, even the aluminum film pad is broken by the probe structure and the electrode as the contact object is damaged. Furthermore, as the surface roughness is larger, upon pressing the bump contact against the contact object, the metal of the contact object is adhered to surface grooves formed among the projections and is left there. Under the circumstances, the surface roughness is preferably specified by Rmax within a range from 0.1 to 0.5 xcexcm and Ra within a range from 0.05 to 0.25 xcexcm.
A ratio of the above-mentioned Rmax to Ra (Rmax/Ra) exceeds 10 and then causes an undesirable state to occur because the surface roughness is widely varied. Specifically, undesirably high projections often appear and are weak in strength. Thus, such projections are poor in durability. A ratio of Rmax/Ra becomes smaller than 2, which results in a smaller variation of the surface roughness, but this is hardly achievable in terms of manufacture. The ratio Rmax/Ra should therefore preferably be at least 2.
Even when the surface oxide film causes no problem, Rmax, Ra and Rmax/Ra should preferably be within the aforementioned ranges because a large contact area is available, with a lower contact resistance, and a stable contact is achieved.
According to configuration 2, the projection spacing (distance between projections associated with contact) is within a range of from 0.1 to 0.8 xcexcm. The projection has a shape of a pitch or spacing that does not exceed Rmax. The projection thickness (thickness at xc2xd height) is at least ⅓ the projection spacing. By satisfying these conditions, it is possible to achieve an excellent durability against repetition of contact with the contact object, and to maintain a stable contact resistance. The term xe2x80x9cprojection pitchxe2x80x9d or xe2x80x9cprojection spacingxe2x80x9d as used herein means the distance between centers of projections associated with contact. The projection center may be either an apex of each projection or a size center of a size determined by a bottom contour of the projection.
Herein, the projection thickness is defined with reference to FIG. 2. A single projection is assumed to be observed on the surface condition of a bump contact by the use of a scanning electron microscope (SEM). In this event, the projection thickness is defined by a size measured at a half height of the projection. Specifically, a curve of the half height is drawn as a dotted line in FIG. 2 by plotting middle or half points between an apex or center of the projection to be contacted and a bottom contour of the projection.
As mentioned in conjunction with configuration 3, when a convex/concave layer (projections) is formed by aggregating fine grains on the surface of the bump contact, a maximum height of each aggregation of fine grains is used as the apex of the projections associated with contact. The bottom of each aggregation of the fine grains is used as the projection bottom when the projection thickness is determined.
At any rate, a high projection density can be obtained by providing one or more projections per xcexcm2. By obtaining a high projection density, a wider contact area is available upon contact with the DUT, and this makes it possible to obtain a stable contact resistance. The projection density should preferably be within a range from at least 1 to up to 50 per xcexcm2, or more preferably, from at least 1 to up to 30 per xcexcm2, or still more preferably, from at least 1 to up to 10 per xcexcm2. The projection herein used means a projection associated with contact, and in the case of a convex/concave layer (projection) formed by the aggregations of fine grains as in configuration 3, each aggregation of fine grains is deemed to be a single projection. An excessively high projection density should be avoided because it leads to a smaller projection thickness and a poorer durability of the projection.
According to configuration 3, aggregations of fine grains permits formation of a dense film. Such a dense film can increase an area adhered to the basic shape portion and can form a stable convex/concave layer having a high projection density. Even when the apex portion of the projection itself is large in size, exposure of fine grains on the surface makes it possible to easily break the oxide film of the contact object.
The convex/concave layer comprising the aggregations of fine grains is formed on the surface of the bump contact without intermediary of an inert layer. This is favorable because it is possible to avoid a decrease in adhesion of the projection or variation of the projection shape caused by the presence of an inert layer.
When the convex/concave layer is formed by using a strongly acidic plating solution on rhodium, an inconvenience may sometimes be caused by oxidation of the surface of the basic shape portion, a poorer adhesion, or non-uniform growth of the film (occurrence of variation of the projection shape). In such a case, the inconvenience can be avoided by providing an oxidation preventing layer for preventing oxidation of the basic shape portion as described in configuration 10.
A convex/concave layer comprising the aggregations of fine grains may be formed on the surface of an electrode and the like which are operable as a part of an electric circuit provided on the other side, namely, the second principal surface of the insulating substrate. The fine grains on the surface of the bump contact may be formed at random, or in conformity to a rule.
According to configuration 4, the aforementioned bump contact comprises a basic shape portion of a single layer, and a convex/concave layer formed directly on the surface of the basic shape portion. As a result, it is possible to avoid deterioration of adhesion of the projecting portions caused by presence of an intermediate layer of gold or the like under the projecting portions, and to easily manufacture the projecting portions because of absence of an intermediate layer.
A preferable bump structure in the probe structure of the present invention is such that the bump contact comprises the basic shape portion and the convex/concave layer having a surface roughness brought about by mat plating described later.
For the necessity to provide a hardness of the mat-plated surface sufficient to withstand repetition of contact with the contact object on the DUT, the hardness should preferably be within a range of from at least 800 to up to 1,000 Hk.
According to configuration 5, the bump contact has a surface roughness given by Rmax within a range from 0.01 to 0.8 xcexcm, Ra within a range from 0.001 to 0.4 xcexcm and Rmax/Ra within a range from 2 to 10. As a result, advantages similar to those of configuration 1 are available in addition to those of configurations 3 and 4. The more preferable range of surface roughness of the bump contact is the same as in configuration 1.
Advantages similar to those of configuration 2 are additionally accomplished by satisfying the requirements of configuration 2 in addition to those of configuration 5.
According to configuration 6, the size of the fine grains within a range of from 5 to 200 nm ensures availability of the advantages of configuration 3.
The grain size should preferably be within a range of from 5 to 100 nm, or more preferably, from 10 to 50 nm.
The size of the grain aggregation (projection) formed through aggregation of fine grains should preferably be within a range of from 0.02 to 1 xcexcm, or more preferably, from 0.1 to 0.4 xcexcm.
As described as to configuration 7, the hardness of the convex/concave layer which is the surface layer should preferably be within a range of from 800 to 1,000 Hk.
With a hardness under 800 Hk, the surface convex/concave layer can easily break an oxidation film upon contact with the contact object, and with a hardness over 1000 Hk, cracks tend to occur.
The surface convex/concave layer is roughened, and should be resistant to a damage caused by repeated contact with the contact object. The convex/concave layer is therefore required to have a hardness higher than the contact object. By imparting corrosion resistance and controllability of transfer and diffusion of the other metals from the contact object, the contact resistance can be preferably reduced.
When a precious metal is used for the surface convex/concave layer, the precious metal may be a single metal or an alloy thereof. In order to avoid an increase in contact resistance resulting from diffusion of a base metal throughout the entire surface and oxidation, an increase in internal stress caused by organic impurities, and occurrence of cracks, the content of the precious metal should preferably be at least 99%. In the case of an alloy, a typical example is a combination of corrosion-resistant precious metals hardly diffusing such as rhodium and ruthenium.
According to configuration 8, a hardness of the basic shape portion under 100 Hk leads to easy deformation when the bump contact is brought into contact with, and pressed against, the contact object. A hardness over 800 Hk tends to cause easy occurrence of cracks.
The material forming the basic shape portion should preferably have crystallographic compatibility with the material of the electrode forming a part of the electric or conduction circuit (electrode electrically connected to the bump contact). In addition, the material should also have good adhesion and hardly diffusion characteristics. For example, when the material for the electrode forming a part of the electric circuit is copper, a preferable combination for the material for the corresponding basic shape portion is nickel or a nickel alloy.
The basic shape portion should preferably have a smooth hemispherically projecting shape.
According to configuration 9, a bump contact having a strong adhesion to the basis shape portion and a surface roughness (convex/concave layer) is obtained by carrying out mat plating without substantially exposing the surface of the basic shape portion to an atmosphere (for example, by continuously carrying out plating).
The process of applying mat plating to the surface layer without substantially exposing the basis shape portion to the atmosphere is, for example, a process of preventing the bump contact from contacting with the atmosphere during a predetermined period. The predetermined period lasts from forming the basic shape portion by plating up to application of mat plating to the surface layer. More specifically, the process comprises the steps of setting an insulating substrate on a jig, putting the same in a plating vessel, forming a basic shape portion by plating, then, rinsing off the plating solution, used for forming the basic shape portion, adhering to the plating jig while taking out the jig from the plating vessel for the next step of surface layer plating, and continuously spraying pure water onto the bump contact prior to mat plating of the surface layer and before and after the treatment so that a water film always covers the bump contact. Thus, the bump contact is prevented from being in contact with the atmosphere during the period from the end of forming of the basic shape portion through entrance thereof in the plating solution for mat plating. The bump contact is not allowed to come into contact with the atmosphere not only by means of the water film, but also during the step of surface activation of the basic shape portion through a sulfate treatment. When two kinds of plating are carried out in succession in the same vessel, as well, pure water is always continuously sprayed onto the bump contact during rising of the plating solution, the plating pretreatment, and rinsing, so as to prevent the bump contact from coming into contact with the atmosphere.
The mat plating means a plating which is effective to achieve a surface condition in which the surface is not glossy and not smooth, and has a property of diffused reflection rather than mirror surface reflection.
When the bump contact of the basic shape portion is exposed to the atmosphere prior to forming a surface layer plating film, the surface of the contact is in inert state, and this deteriorates adhesion with the surface layer plating formed in the following step. This inert condition of the contact surface is considerable particularly when using nickel or a nickel alloy, and activation through a pretreatment is difficult.
By applying mat plating without exposing the surface of the basic shape portion to the atmosphere, strong adhesion between the different materials is ensured, and this gives a plating film which hardly comes off even after repeated contact with the contact object.
In order to prevent the surface of the basic shape portion from being exposed to the atmosphere, as in configuration 10, an oxidation preventing layer for preventing oxidation of the basic shape portion may be formed prior to mat plating. When the oxidation preventing layer is thick, a decrease in adhesion of the projection or variation of the projection shape would be produced. The thickness should therefore be the smallest possible. As in configuration 11, the thickness of the oxidation preventing film should preferably be within a range of from 0.001 to 0.05 xcexcm. Methods of forming an oxidation preventing film having a thickness of this order include stroke plating. The material for the oxidation preventing film should be gold, silver or palladium.
Particularly, when the material for the convex/concave layer is rhodium formed with a strongly acidic plating solution, the surface of the basic shape portion may be oxidized, adhesion may be deteriorated, or the film growth may become non-uniform (variation of the projection shape may be produced), even if the surface is not substantially exposed to the atmosphere. Formation of the oxidation preventing film is therefore effective. 0
According to configuration 12, by forming the surface convex/concave layer with a relatively low plating current density (specifically, within a range of from 0.1 to 1.0 A/dm2), it becomes easier to control the surface roughness condition. Even when providing a plurality of bump contacts on a substrate, therefore, it is possible to make the surface condition of each bump contact closer to a uniform state, and reduce variation of contact condition between the contact object and each bump contact.
By always keeping a current density of mat plating constant, the current density is stabilized, and variation of the projection shape is never produced.
By varying the current density, the amount of polishing agent, and the plating material in the plating step, furthermore, it is possible to control the surface roughness condition and the projection density, and achieve a surface roughness suitable to break through the oxide film of the electrode section.
According to configuration 13, rhodium used as the material for mat plating gives strong adhesion, and makes it difficult for the plating film to come off even after repeated contact with the contact object. By varying the current density, the amount of polishing agent, and the plating material in the plating step, it is possible to accurately control the surface roughness condition and the projection density.
According to configuration 14, contact resistance can be kept low from the initial stage of contact, and also can be maintained at a low level even upon the lapse of a period of time in a heated state. This is very effective when a burn-in test is carried out, in addition to the advantages described as to configurations 1 to 8. In this case, a bump is subjected to mat rhodium plating on the surface of the basic shape portion of nickel alone or a nickel alloy. No inert layer is deposited in the bump.
Similarly, as in configuration 15, by having a bump subjected to gold strike plating and mat rhodium plating on the surface of the basic shape portion of nickel alone or a nickel alloy, contact resistance is low from the initial stage of contact, and contact resistance can be maintained at a low level even upon the lapse of a period of time even in a heated state. This is effective on carrying out the burn-in test, in addition to the advantages described as to configurations as to configurations 1 to 8. For the purpose of simplifying the manufacturing process, configuration 14 is preferable. However, on using a strongly acidic plating solution such as that in mat rhodium plating, it is possible to ensure prevention of oxidation of the surface of the surface layer of nickel alone or a nickel alloy, and avoidance of a decrease in adhesion of the projections or variation of the projection shape. This results in an improved reliability.
According to the present invention, it is possible to limit the variation in contact resistance to up to 1 xcexa9 after contacts are repeated 300 times.